80 INVERTEBRATE ZOOLOGY 



very distinctly different types of ocelli. Those of one type, 

 exemplified by the median ocelli of insects (Fig. 48), are but 

 little less complicated than the compound eyes, while the lateral 

 ocelli are much simpler. 



The simple, unpaired median eye of the lower crustaceans 

 shows many features in common with the eyes of the flatworms. 



There are numerous modifications for auditory and balancing 

 functions in invertebrates but most of these are recognizable as 

 some sort of otocyst or statocyst (Fig. 39), which depends upon 

 the movement of a free or suspended body, the otolith or statolith, 

 against the walls of the vesicle for stimulating the central nervous 

 system and causing the organism to right itself. In some of the 

 insects is found a highly specialized auditory organ in which a 

 tympanum receives sound waves and transmits the stimulus 

 through an underlying space to the nerve endings in the sensory 

 epithelium. 



References 



Dahlgren, U. and Kepner, W. A. 1908. "A Text-book of the Principles 

 of Animal Histology." New York, Macmillan. 



Kellicott, W. E. 1913. "A Textbook of General Embryology." New- 

 York, Holt. 



KoRSCHELT, E. and Heider, K. 1895-1900. "Textbook of the Embryology 

 of Invertebrates." (English translation.) London, Sonnenschein. 



McBride, E. W. 1914. "Text-book of Embryology." Vol. 1, Inverte- 

 brata. London, Macmillan. 



Schneider, K. C. 1902. "Lehrbuch der Vergleichenden Histologic der 

 Tiere." Jena, G. Fischer. 



